The present invention relates to a display device, particularly a reflection type liquid crystal display device using a liquid crystal material.
A liquid crystal display device is widely used in such various applications as a display, because it is characterized by thinness and lightness. A liquid crystal display device is a light-receiving type display device that provides display by varying an intensity of light transmitted therethrough without emitting light by itself, and can be driven by an effective voltage of several volts. Thus, when a liquid crystal display device with a reflector plate provided under the liquid crystal display device is employed as a reflection type display device for providing display by means of reflection of external light, a display device significantly low in power consumption can be obtained.
A reflection type color liquid crystal display device of prior art comprises liquid crystal cell provided with color filters and a pair of polarizing filters with the liquid crystal cell between them. The color filters are provided in one of substrates of the liquid crystal cell, and a transparent electrode is further provided on the color filters that are formed on the substrate. By applying a voltage to the liquid crystal cell, liquid crystal molecules of the liquid crystal material is changed in alignment, and a transmittance of light through the filters of various colors is thereby changed, thus color display is achieved.
A transmittance of light through one piece of polarizing plate is about 45% at the maximum. Then, a transmittance of light polarized parallel to an absorption axis of the polarizing film is about 0%, and that of light polarized vertically is about 90%. In a reflection type liquid crystal display device with two polarizing plates, the light passes through the polarizing films four times before it comes out. Therefore, if absorption of light by the color filters is disregarded, a light utilization factor of the liquid crystal display device is expressed by:
(0.9)4xc3x9750%=32.8%
In other words, the light utilization factor is about 33% even in a black-and-white panel, and it is the maximum value of light utilization factor.
Then, in order to provide a brighter display, it is suggested to provide a polarizing film only in an upper side of a liquid crystal cell so that the liquid crystal cell is sandwiched between the polarizing film and a reflector plate (see, for example, Japanese Laid-open Patent Publication Nos. 7-146469 and 7-84252). In such case, the light passes the polarizing film twice. If absorption of light by the color filter is disregarded, a light utilization factor is expressed by:
(0.9)2xc3x9750%=40.5%
Thus, it can be expected that the light utilization factor is increased by about 23.5% at the maximum in comparison with a liquid crystal display device with two polarizing films.
However, a light utilization factor of reflection type liquid crystal display device does""t depend only on optimization of components. In a reflection type liquid crystal display device, display is provided not by means of such certain light source as back light system as in the case of a transmission type liquid crystal display, but by using an external light that is changed in various manners due to environmental conditions. In an external light, direct and indirect lights or parallel and scattered lights are contained in an mixed manner. Further, in an external light, the direction of incident light can be changed variously. Therefore, it is required that a reflection type liquid crystal display device provides a light utilization factor that is maintained at or above a certain value even when conditions of an external light is changed.
It has also been a problem of conventional reflection type liquid crystal display device that display characteristics are varied by conditions of an external light. The problem is partly caused due to a narrow viewing angle. When the viewing angle of liquid crystal display device is narrow, if the ratio of external light incident in the oblique direction is high, a utilization factor of effective light coming out to a general observation position is lower.
It is an object of the invention to provide such reflection type liquid crystal display device with only a polarizing plate that provides a wide viewing angle, and is capable of assuring a light utilization factor at or above a certain value so that bright display is provided even when conditions of an external light are varied.
A reflection type liquid crystal display device according to the invention comprises:
a liquid crystal cell provided with an upper substrate, a lower substrate and a liquid crystal layer located between the upper and lower substrates;
a polarizing film provided in a side of the upper substrate of the liquid crystal cell;
light reflecting means provided in a side of the lower substrate of the liquid crystal cell; and
an optical member provided between the polarizing film and the liquid crystal cell, and having a retardation axis when it is viewed in the normal direction,
wherein an angle between an absorption axis of the polarizing film and the retardation axis of the optical member is within at least one of ranges of:
(i) about 88xc2x0 to about 92xc2x0 and (ii) about xe2x88x922xc2x0 to about +2xc2x0 .
It is particularly desirable that a retardation value in relation to the normal direction of the optical member is within a range of about 50 nm to about 500 nm.
According to such constitution, a reflection type liquid crystal display device having a characteristic of wide viewing angle, and capable of assuring a light utilization factor at or above a certain level for bright display even when conditions of an external light are varied.